There is still tons of ceremony for mail in ballots, it just happens asynchronously to the voter making selections. That's true of paper ballots as well, as significant parts of the ceremony happens after the poll closes during the count.
The biggest problem is that email and online voting suffer from a lack of anonymity.
In person: voter eligibility is confirmed and marked executed, voter makes selections, ballot securely placed in box without being read.
Mail: voter eligibility is confirmed, ballot sent, voter makes selections, voter mails ballot back secured against tampering, voter eligibility is re-confirmed and marked executed, ballot securely placed in box without ever being read.
In both cases all the poll worker steps are overseen by election scrutineers who can confirm all legal requirements were adhered to, in particular that no one knows who anyone voted for, just that the vote was collected.
Email cannot replicate the separation of the voter eligibility verification with the ballot counting. No matter what you do there will always be pathways to link a specific voter to the ballot they cast.